1. Field of the Invention
The present invention relates to an optical transmitting apparatus and an optical communication system, and in particular, relates to optical transmitting apparatus for transmitting an optical signal using PSK modulation and an optical communication system comprising such optical transmitting apparatus.
2. Description of the Related Art
Development of a practical implementation of an optical transmitting apparatus aiming to establish a high capacity and long distance optical transmission system has been awaited in recent years. Particularly, expectations for implementation of an optical transmitting apparatus, which employs an optical modulation technique adequate for high capacity and long-distance, to an actual system are growing high. In order to meet with expectations, optical transmission systems using phase shift keying such as DPSK (Differential Phase Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) are envisioned.
FIG. 1 is a diagram showing a configuration of optical transmitting apparatus, which is heretofore known, for transmitting a QPSK signal or a DQPSK signal. In optical transmitting apparatus 2000 shown in FIG. 1, an optical source (LD) 2001 generates an optical continuous wave (CW). An optical splitter 2002 splits the optical CW and guides to arms 2003 and 2004. In the arms 2003 and 2004, phase modulators 2005 and 2006 are configured respectively. Both phase modulators 2005 and 2006 are Mach-Zehnder interferometers. The phase modulator 2005 performs phase modulation of the optical CW using data 1, and the phase modulator 2006 performs phase modulation of the optical CW using data 2. In DQPSK, the data 1 and data 2 are encoded by a DQPSK pre-coder. An optical coupler 2007 couples modulated signals obtained from the phase modulators 2005 and 2006. A phase shifter 2008 causes the phase difference of π/2 between an optical signal guided to the optical coupler 2007 via the arm 2003 and an optical signal guided to the optical coupler 2007 via the arm 2004. By the above configuration, a QPSK optical signal or a DQPSK optical signal is generated. Details of the configuration and the operation of DQPSK optical transmitting apparatus are described, for example, in Patent Document 1 (Japanese Patent Application Laid-open Publication No. 2004-516743 (WO2002/051041 or US2004/0081470)).
As a related art, Patent Document 2 (Japanese Patent Application Laid-open Publication No. 2004-318052) describes a technology for appropriately correcting DC bias in each optical modulator unit of SSB (Single Side-Band) optical modulator apparatus comprising a plurality of the optical modulator units, during normal operation of the modulator. In the SSB optical modulator apparatus also, a phase shifter for causing the phase difference of π/2 in optical signals propagated in a pair of arms is configured.
As other related arts, Patent documents 3 (Japanese Patent Application Laid-open Publication No. H03-251815) and Patent document 4 (Japanese Patent Application Laid-open Publication No. H09-261207) describe a configuration to adjust the operation of modulator apparatus by using a low-frequency signal.
In the optical transmitting apparatus shown in FIG. 1, it is necessary for the amount of phase shift provided by the phase shifter 2008 (“π/2” in the QPSK and the DQPSK) to be set accurately to attain favorable reception quality. However, the amount of phase shift may sometimes deviate from a prescribed value due to thermal change and aged degradation etc.
Nevertheless, in the conventional technologies, a configuration for adjusting the amount of phase shift of the phase shifter in response to thermal change or aged degradation etc. has not been suggested.